I built a 4:1 balun so I could start playing with an OCF inverted-V dipole. I followed the
ZS1AN hybrid balun article which combines a voltage transformer and a choke. I used the
same cores he did, even though he admits they're not optimal. So now
that it's built, I'm thinking about how to optimize the design for the
next build.
Where I'm having trouble is picking the right cores. In this case, the
transformer is 8 turns of #16 (he used #14) enamel coated wire wound
over an FT240-61. The choke is 8 turns of #16 over an FT114-61. All
the windings are inside Teflon sleeves.
On the analyzer, it turned out to be between 3.4:1 and 3.6:1 across the
ham bands from 80m to 10m. That's into a 200 ohm resistor. I
guess that puts me in the right ballpark for an OCF.
A tiny bit of knowledge is a dangerous thing. I was poking
around with some core calculator utilities and I'm thinking that
FT114-61 choke might saturate at 100 watts especially if I'm driving the antenna on bands that it doesn't handle naturally. Also, I suppose it'd be better to align the toroids 90
degrees from each other so they don't share the same orientation.
So I'm wondering if I'd be better off with an FT240-43 core for the
transformer and and FT240-77 for the choke. But really, I'm lost here.
What say you RF gurus?
BTW, 100 watts max and I'm interested in whatever bands I can make work from 80m to 10m.